How it Works
VDSL works similarly to ADSL, dividing the available band into channels, and testing the signal-to-noise ratio of each channel to determine its maximum speed, process known as DMT (Discrete Multi-Tone). Read our ADSL tutorial for a more in-depth explanation of this process.
The main difference between ADSL and VDSL is the bandwidth available. While ADSL and ADSL2 have available a 1,104 kHz band, which is divided into 256 channels, and ADSL2+ has available a 2,208 kHz band, divided into 512 channels, VDSL can use a band of either 8 MHz, 12 MHz, 17 MHz, or 30 MHz, as shown in the table below. The use of these wider bands allows far higher transfer rates.
|Profile||Bandwidth||Channels||Channel Size||Max. Downstream Speed||Max. Upstream Speed||Downstream Power||Upstream Power|
|8a||8,832 kHz||2,048||4.3125 kHz||50 Mbps||NA||+17.5 dBm||+14.5 dBm|
|8b||8,832 kHz||2,048||4.3125 kHz||50 Mbps||NA||+20.5 dBm||+14.5 dBm|
|8c||8,832 kHz||2,048||4.3125 kHz||50 Mbps||NA||+11.5 dBm||+14.5 dBm|
|8d||8,832 kHz||2,048||4.3125 kHz||50 Mbps||NA||+14.5 dBm||+14.5 dBm|
|12a||12,000 kHz||2,783||4.3125 kHz||68 Mbps||NA||+14.5 dBm||+14.5 dBm|
|12b||12,000 kHz||2,783||4.3125 kHz||68 Mbps||NA||+14.5 dBm||+14.5 dBm|
|17a||17,664 kHz||4,096||4.3125 kHz||100 Mbps||50 Mbps||+14.5 dBm||+14.5 dBm|
|30a||30,000 kHz||3,479||8.625 kHz||200 Mbps||100 Mbps||+14.5 dBm||+14.5 dBm|
The maximum speeds are theoretical and actual speeds will depend on the length of the copper wires. Initially, 100 Mbps speeds could only be achieved on FTTH deployment, however, with a noise-cancelling technology called “vectoring,” now it is possible to achieve this speed with regular copper wires.
What is interesting about VDSL is that the division of the band is done in a way to keep VDSL 100% compatible with ADSL. The beginning of the band is divided just like ADSL2+, and the rest of the band is divided into several intercalated upstream and downstream bands. The division of the available spectrum varies depending on the VDSL2 profile and also on the band plan followed by the service provider. On this link you can see detailed diagrams of all possible deployments.